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Message-ID: <874kocmqqx.wl-maz@kernel.org>
Date: Sat, 05 Sep 2020 12:01:42 +0100
From: Marc Zyngier <maz@...nel.org>
To: Jianyong Wu <jianyong.wu@....com>
Cc: netdev@...r.kernel.org, yangbo.lu@....com, john.stultz@...aro.org,
tglx@...utronix.de, pbonzini@...hat.com,
sean.j.christopherson@...el.com, richardcochran@...il.com,
Mark.Rutland@....com, will@...nel.org, suzuki.poulose@....com,
steven.price@....com, linux-kernel@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
kvm@...r.kernel.org, Steve.Capper@....com, justin.he@....com,
nd@....com
Subject: Re: [PATCH v14 08/10] ptp: arm64: Enable ptp_kvm for arm64
On Fri, 04 Sep 2020 10:27:42 +0100,
Jianyong Wu <jianyong.wu@....com> wrote:
>
> Currently, there is no mechanism to keep time sync between guest and host
> in arm64 virtualization environment. Time in guest will drift compared
> with host after boot up as they may both use third party time sources
> to correct their time respectively. The time deviation will be in order
> of milliseconds. But in some scenarios,like in cloud envirenment, we ask
> for higher time precision.
>
> kvm ptp clock, which choose the host clock source as a reference
> clock to sync time between guest and host, has been adopted by x86
> which makes the time sync order from milliseconds to nanoseconds.
>
> This patch enables kvm ptp clock for arm64 and improve clock sync precison
> significantly.
>
> Test result comparisons between with kvm ptp clock and without it in arm64
> are as follows. This test derived from the result of command 'chronyc
> sources'. we should take more care of the last sample column which shows
> the offset between the local clock and the source at the last measurement.
>
> no kvm ptp in guest:
> MS Name/IP address Stratum Poll Reach LastRx Last sample
> ========================================================================
> ^* dns1.synet.edu.cn 2 6 377 13 +1040us[+1581us] +/- 21ms
> ^* dns1.synet.edu.cn 2 6 377 21 +1040us[+1581us] +/- 21ms
> ^* dns1.synet.edu.cn 2 6 377 29 +1040us[+1581us] +/- 21ms
> ^* dns1.synet.edu.cn 2 6 377 37 +1040us[+1581us] +/- 21ms
> ^* dns1.synet.edu.cn 2 6 377 45 +1040us[+1581us] +/- 21ms
> ^* dns1.synet.edu.cn 2 6 377 53 +1040us[+1581us] +/- 21ms
> ^* dns1.synet.edu.cn 2 6 377 61 +1040us[+1581us] +/- 21ms
> ^* dns1.synet.edu.cn 2 6 377 4 -130us[ +796us] +/- 21ms
> ^* dns1.synet.edu.cn 2 6 377 12 -130us[ +796us] +/- 21ms
> ^* dns1.synet.edu.cn 2 6 377 20 -130us[ +796us] +/- 21ms
>
> in host:
> MS Name/IP address Stratum Poll Reach LastRx Last sample
> ========================================================================
> ^* 120.25.115.20 2 7 377 72 -470us[ -603us] +/- 18ms
> ^* 120.25.115.20 2 7 377 92 -470us[ -603us] +/- 18ms
> ^* 120.25.115.20 2 7 377 112 -470us[ -603us] +/- 18ms
> ^* 120.25.115.20 2 7 377 2 +872ns[-6808ns] +/- 17ms
> ^* 120.25.115.20 2 7 377 22 +872ns[-6808ns] +/- 17ms
> ^* 120.25.115.20 2 7 377 43 +872ns[-6808ns] +/- 17ms
> ^* 120.25.115.20 2 7 377 63 +872ns[-6808ns] +/- 17ms
> ^* 120.25.115.20 2 7 377 83 +872ns[-6808ns] +/- 17ms
> ^* 120.25.115.20 2 7 377 103 +872ns[-6808ns] +/- 17ms
> ^* 120.25.115.20 2 7 377 123 +872ns[-6808ns] +/- 17ms
>
> The dns1.synet.edu.cn is the network reference clock for guest and
> 120.25.115.20 is the network reference clock for host. we can't get the
> clock error between guest and host directly, but a roughly estimated value
> will be in order of hundreds of us to ms.
>
> with kvm ptp in guest:
> chrony has been disabled in host to remove the disturb by network clock.
>
> MS Name/IP address Stratum Poll Reach LastRx Last sample
> ========================================================================
> * PHC0 0 3 377 8 -7ns[ +1ns] +/- 3ns
> * PHC0 0 3 377 8 +1ns[ +16ns] +/- 3ns
> * PHC0 0 3 377 6 -4ns[ -0ns] +/- 6ns
> * PHC0 0 3 377 6 -8ns[ -12ns] +/- 5ns
> * PHC0 0 3 377 5 +2ns[ +4ns] +/- 4ns
> * PHC0 0 3 377 13 +2ns[ +4ns] +/- 4ns
> * PHC0 0 3 377 12 -4ns[ -6ns] +/- 4ns
> * PHC0 0 3 377 11 -8ns[ -11ns] +/- 6ns
> * PHC0 0 3 377 10 -14ns[ -20ns] +/- 4ns
> * PHC0 0 3 377 8 +4ns[ +5ns] +/- 4ns
>
> The PHC0 is the ptp clock which choose the host clock as its source
> clock. So we can see that the clock difference between host and guest
> is in order of ns.
>
> Signed-off-by: Jianyong Wu <jianyong.wu@....com>
> ---
> drivers/clocksource/arm_arch_timer.c | 24 +++++++++++++
> drivers/ptp/Kconfig | 2 +-
> drivers/ptp/ptp_kvm_arm64.c | 53 ++++++++++++++++++++++++++++
> 3 files changed, 78 insertions(+), 1 deletion(-)
> create mode 100644 drivers/ptp/ptp_kvm_arm64.c
>
> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> index d55acffb0b90..aaf286e90092 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -1650,3 +1650,27 @@ static int __init arch_timer_acpi_init(struct acpi_table_header *table)
> }
> TIMER_ACPI_DECLARE(arch_timer, ACPI_SIG_GTDT, arch_timer_acpi_init);
> #endif
> +
> +#if IS_ENABLED(CONFIG_PTP_1588_CLOCK_KVM)
> +#include <linux/arm-smccc.h>
> +int kvm_arch_ptp_get_crosststamp(unsigned long *cycle, struct timespec64 *ts,
> + struct clocksource **cs)
> +{
> + struct arm_smccc_res hvc_res;
> + ktime_t ktime;
> +
> + /* Currently, linux guest will always use the virtual counter */
> + arm_smccc_1_1_invoke(ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID,
> + ARM_PTP_VIRT_COUNTER, &hvc_res);
> + if ((long long)(hvc_res.a0) < 0)
> + return -EOPNOTSUPP;
> +
> + ktime = (long long)hvc_res.a0;
> + *ts = ktime_to_timespec64(ktime);
> + *cycle = (long long)hvc_res.a1;
> + *cs = &clocksource_counter;
> +
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(kvm_arch_ptp_get_crosststamp);
> +#endif
> diff --git a/drivers/ptp/Kconfig b/drivers/ptp/Kconfig
> index 942f72d8151d..127e96f14f89 100644
> --- a/drivers/ptp/Kconfig
> +++ b/drivers/ptp/Kconfig
> @@ -106,7 +106,7 @@ config PTP_1588_CLOCK_PCH
> config PTP_1588_CLOCK_KVM
> tristate "KVM virtual PTP clock"
> depends on PTP_1588_CLOCK
> - depends on KVM_GUEST && X86
> + depends on KVM_GUEST && X86 || ARM64 && ARM_ARCH_TIMER && ARM_PSCI_FW
> default y
> help
> This driver adds support for using kvm infrastructure as a PTP
> diff --git a/drivers/ptp/ptp_kvm_arm64.c b/drivers/ptp/ptp_kvm_arm64.c
> new file mode 100644
> index 000000000000..961abed93dfd
> --- /dev/null
> +++ b/drivers/ptp/ptp_kvm_arm64.c
> @@ -0,0 +1,53 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Virtual PTP 1588 clock for use with KVM guests
> + * Copyright (C) 2019 ARM Ltd.
> + * All Rights Reserved
> + */
> +
> +#include <linux/kernel.h>
> +#include <linux/err.h>
> +#include <asm/hypervisor.h>
> +#include <linux/module.h>
> +#include <linux/psci.h>
> +#include <linux/arm-smccc.h>
> +#include <linux/timecounter.h>
> +#include <linux/sched/clock.h>
> +#include <asm/arch_timer.h>
> +
> +int kvm_arch_ptp_init(void)
> +{
> + struct arm_smccc_res hvc_res;
> +
> + arm_smccc_1_1_invoke(ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID,
> + &hvc_res);
> + if (!(hvc_res.a0 | BIT(ARM_SMCCC_KVM_FUNC_KVM_PTP)))
> + return -EOPNOTSUPP;
> +
> + return 0;
What happens if the ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID function
isn't implemented (on an old kernel or a non-KVM hypervisor)? The
expected behaviour is that a0 will contain SMCCC_RET_NOT_SUPPORTED,
which is -1. The result is that this function always returns
"supported". Not an acceptable behaviour.
> +}
> +
> +int kvm_arch_ptp_get_clock_generic(struct timespec64 *ts,
> + struct arm_smccc_res *hvc_res)
Why isn't this static?
> +{
> + ktime_t ktime;
> +
> + arm_smccc_1_1_invoke(ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID,
> + hvc_res);
> + if ((long long)(hvc_res->a0) < 0)
> + return -EOPNOTSUPP;
Really? What if the cycle counter is a full 64 bit value, as it is
*mandated* on ARMv8.6? It means that the counter is now invalid for
half the lifetime of the system. Not acceptable either.
> +
> + ktime = (long long)hvc_res->a0;
> + *ts = ktime_to_timespec64(ktime);
> +
> + return 0;
> +}
> +
> +int kvm_arch_ptp_get_clock(struct timespec64 *ts)
> +{
> + struct arm_smccc_res hvc_res;
> +
> + kvm_arch_ptp_get_clock_generic(ts, &hvc_res);
> +
> + return 0;
> +}
> --
> 2.17.1
>
>
It is now obvious that the API between kernel and hypervisor is pretty
busted, and it goes beyond the 32bit support. I wish you paid more
attention to this kind of detail.
M.
--
Without deviation from the norm, progress is not possible.
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